It is known that dark-grown (etiolated) plants undergo spectral changes under the influence of light. By brief illumination of the etiolated leaves the initial absorption maximum in the red region of the spectrum is irreversibly shifted from about 647 nm to about 676 nm and the initial low temperature fluorescence emission maximum in the red region of the spectrum is irreversibly shifted from about 657 nm to about 688 nm.
The protochlorophyll(ide) which has accumulated in the dark-grown plants and which is the direct precursor of the chlorophyll found in normal green chloroplasts is responsible for the above phenomenon by photoreduction of the protochlorophyll(ide) to chlorophyll(ide).
Essential for the photoreduction of the protochlorophyll(ide) to chlorophyll(ide) is a specific binding with an apoprotein. This binary complex of protochlorophyll(ide) with apoprotein has been termed protochlorophyll(ide) holochrome.
The isolated protochlorophyll(ide) holochrome extracted from the etiolated plants as described in the literature is unstable in the sense that it is photoactive only if it is kept in the dark at a temperature below 0.degree. C., and the photoactivity is only retained for an appreciable length of time, say some weeks or months, if the holochrome is stored at a temperature appreciably below 0.degree. C.